Pretreating thorium for electroplating



2,897,124 PRE'PREATING THORIUM non EIJECTROPLATING .lohn G. Beach and Glenn R. Schaer, Columbus, Ohio,

assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application October 19, 1954 Serial No. 463,338 2 Claims. Cl. 204-15 The present invention relates to the electroplating of metals on a thorium surface and in particular is concerned with a method for the pretreatment of a thorium surface prior to electroplating on said surface.

Thorium is a soft, malleable metal having'a facecentered cubic lattice. Because of the electropositive nature of thorium and its great tendency to form canbides, nitrides and silicides at high temperatures, itis' quite subject to corrosion. Therefore for most applications it is desirable to protect the surface of thorium objects with a coating of other metals. Now, while corrosionresistant coatings on metals are usually best applied by electroplating, it has been found that the conventional electroplating methods which are used on other basis metals are unsuitable for use with thorium. Even the methods for electroplating on uranium which closely resembles thorium chemically, and magnesium and aluminum which resemble thorium metallurgically were most unsatisfactory with thorium. It'was therefore necessary to have a new approach to the problem of electroplating on thorium.

It is an object of the present invention to provide an efficient method for preparing a thorium surface for electroplating. i I

Additional objects of the present invention will be apparent to those skilled in theart from the description which follows. i

The smoothness, porosity, adhesion and "uniformity of electroplated coatings on metals are usually no better than the surfaces to which they are applied. Accordingly, the preparation of the thorium surface is fundamental in obtaining a satisfactory quality in the final product. The process of the present invention for pretreating thorium surfaces prior to electroplating comprises four steps.

These are (a) a physical cleaning step, (b) an anodic' pickling step, (c) a chemical pickling step, andtd) a water rinse. v

The initial step of the present process comprises a mechanicalcleaning of the thorium surface by abrasion, or preferably by vapor blasting. This step removes any loose thorium scale which may be present on the surface. Other methods of removing the scale haje been tried, such as chemically pickling in a nitric-hydrofluosilicic acid bath, and anodic polishing in a phosphoric and/ or sulfuric acid solution, but these other methods resulted in a thorium surface which was resistant to the anodic pickling in hydrochloric-acid which terns second step of the present process.

Following the c'leaningstep, the thoriu'm'is then giveh an anodic pickling step in hydrochloric acid. (It has'been found that thorium surfaces are suitably prepared by pickling in a to 15% and preferably '(volume) aqueous hydrochloric acid bath with a current of 125 to 250 amps/sq. ft. for 3 to 5 minutes at room temperature. It will, of course, be understood that any one of these factors of hydriichlo'fi'c acid concentration, current, time 'or temperature may be varied by suitable adjustment of one or more of the other factors. For example, the

Patented July as, 1959 15% by volume. It will be understood that suitable vari ations may be made inthe concentration, time and temperature 'by adjusting the other variables to compensate therefor.

Following the chemical pickle, the article is then rinsed in Water. Tap water is suitable for this rinse. These four steps comprise the pretreatment for the thorium article.

While we do not wish to be bound by any theory advanced, it is believed that the anodic pickling step; results in the formation of a coating of thorium perchlorate on the thorium surface.

' coating is then converted to a thorium sulfate film by the chemical pickling step. This film is assumed to protect the thorium surface from oxidation until the cathodic action during plating reduces the sulfate film and permits deposition of the plated metal directly on the thorium metal surface. ported' by the fact that the entire sequence of steps is necessary if the plating step is to be carried out from an alkaline solution. Where the plating step is to be carried out from an acid solution, the sulfuric acid chemical pickle may be omitted. It is presumed that in that case be obtained froman alkaline pyropliosphate bath cont-aining sodium citrate. This apparent discrepancy is possibly explained by the assumption that thorium hydroxide forms a soluble complex with sodium citrate, thus exposing a clean thorium surface. However, by following the complete sequence of the thorium pretreatrnerit steps of the process of this invention, adherent electroplates of various metals may be obtained on thorium from either an acid or an alkaline batha 7 Now that the pretreatment processrhas been generally.

described, it may be further illustrated by the followin specific example. 7 Example A thorium ingot was hot-forged in air at 1500 F. to a 1 inch by 3 inch bar,then hot-rolled in air at 1500 F. to mils thickness and vapor-blasted to free the bar of scale. The bar was then anodically pickled in a 10% by volume hydrochloric acid solution for 4 minutes at 200 amps/sq. ft. with the temperature of the solution at room temperature. Following the anodic pickling step, the bar was chemically pickled in a 10% by volume.

sulfuric acid solution maintained at room temperature for 4 minutes. The bar was then thoroughly rinsed with tap water. After this pretreatment the thorium was ready for electroplating.

Various metals have bee'nplated to throium following the pretreatiiieht of the present process. These metals inc1ude"aluminum, iinc, chromium, nickel, lead, copper,

silver, indium, gold and rhodium. Thorium prepared by This thorium perchlorate This theoryis somewhat su'pthe present method can be successfully electroplated in most types of commercial baths. Certain bath constituents or conditions, however, destroy the pretreated surfaces. For example, halogen or halogen-containing acids, and electrolytes operated at a pH lower than 4.0 are detrimental to the surface produced by the present pretreatment. Also a high-temperature chromic acid solution for depositing low-contraction chromium is unsuitable for plating on the pretreated thorium surface. However, for all commonly electrodeposited metals, at least one type of bath has been found satisfactory for direct deposition on throium following the pretreatment of the present invention. Representative baths which have been found to give suitable electroplates on thorium following the present pretreatment are hereafter described. An oscillating cathode with agitation of 2 feet per minute was used except when noted.

Aluminum was electroplated on pretreated thorium by a Hurly-Wier process utilizing a nonaqueous aluminum chloride-ethyl pyridinium bromide eutectic electrolyte. A current density of 10 amps/sq. ft. deposited a smooth, ductile, adherent coating 1 mil thick in 4 hours, approximating 100% current efliciency. The Hurly-Wier process is described in US. Patents 2,446,331, 2,446,349 and 2,446,350.

Zinc can be electroplated on pretreated thorium from either an alkaline or an acid bath. An'alkaline bath which can be used consists of 60 grams/liter ZnCN, 72.5 grams/liter NaCN, 52.5 grams/liter NaOH and 0.5 gram/liter Na S. The plating conditions are 120' F. bath temperature, 20 amps/sq. ft. current and a zinc anode. An acid zinc bath which may be used consists of 240 grams/liter ZnSO -7H O, 15 grams/liter NH Cl, 30 grams/liter A1 (SO -18H O, 70 grams/liter and 0.7 gram/liter licorice. The conditions are 90 F a pH of 4.5 and 35 amps/sq. ft.

Normal, hard chromium is electroplated on thorium from the following bath and conditions with lead anodes and without cathode agitation: 250 grams/ liter CrO and 2.5 grams/liter H 80 at 110 F. and 140 amps/sq. ft.

Nickel is successfully electroplated on thorium from an electrolyte comparable to those used to electroplate on such active metals as zinc and aluminum. The characteristic bath is as follows: 143 grams/liter NiSO 7H O, 75 grams/liter MgSO -7H O, 15 grams/liter NH Cl and 15 grams/liter H BO The bath conditions are a pH of 5.6, 90 F., 15 amps/sq. ft. and a nickel anode.

Tin can be electroplated on thorium following the pretreatment of the present invention from a commercial alkaline sodium stannite type of bath. Steel anodes can be used. The bath consists of 90 grams/liter 7.5 grams/liter NaOH, 15 grams/liter NaC H O and 0.5 gram/liter H with temperature and current conditions of 150 F. and 20 amps/sq. ft.

Lead can be electroplated from an alkaline tartrate bath with lead anodes, the bath having the following composition: 75 grams/liter Pb (C H O )OH, 35 grams/liter KNaC H O -4H O, 180 grams/ liter NaOH and 0.5 gram/ liter betaine. The temperature of the bath is 160 F. and the current 25 amps/sq. ft.

Copper is successfully electroplated on thorium following the present pretreatment from an acid copper sulfate bath using copper anodes and with the bath having the following composition: 200 grams/liter CuSO -I-I O and 90 grams/liter H 80 at 90 F. and 40 amps/sq. ft. Other copper baths which may be used include copper pyrophosphate baths and copper cyanide baths.

Silver also can be electroplated from the usual commercial type of silver cyanide bath using silver anodes. A bath composition of 75 grams/liter AgCN, 112 grams/ 4 liter KCN and 22.5 grams/liter K CO with a temperature of 113 F., a pH of 13.0 (with KOH) and a current of 25 amps/sq. ft; is satisfactory.

Indium can be electroplated on the pretreated thoriu from an alkaline cyanide bath using stainless steel anodes. The bath composition and conditions are 30 grams/liter lnCl 90 grams/liter KOH, 90 grams/liter KCN and 30 grams/liter dextrose at 90 F. and amps/sq. ft.

Gold can be successfully electroplated on the pretreated thorium from a cyanide bath using stainless steel anodes. The composition of the bath is 7.4 grams/liter potassium gold cyanide, 10.6 grams/liter KCN, 7.5 grams/liter K PO 1 gram/liter NiSO -6H O and 1 cc. liter NH OH (28%). Temperature and current cond1- ions are 140-150 F. and 1.5 amps/sq. ft.

Samples of thorium pretreated by the present process and then plated with the above metals by the processes described were tested for adherence of the plated metal by several different methods. One test consisted of grinding or filing the edges of the electroplate perpendicular to the interface and in a direction such as to drag the plate away from the thorium. No separation was observed in any of the electroplated samples tested.

A chisel test was tried in which it was attempted to separate the electroplate from the basis metal by chiseling along the interface. Separation by cutting was obtained within the thorium or in the electroplate by this method, but no separation was observed at the interface.

The specimens were then submitted to a bend test in which they were bent around a A-inch mandrel, refiattened, and then bent back again around the mandrel. No separation was apparent for deposits less than 3 to 4 mils thick.

The electroplated zinc, copper, nickel and silver on thorium basis metal were also tested by cyclic heating in argon to successively higher temperatures (at C. intervals for 15 minutes) and quenching in cold water. Bond failure for copper, nickel or silver was first observed after cyclic heating and quenching to 600 or 700 C. Since the thorium metal or alloy remained firmly attached to the separated electroplate, however, the bond failure was considered to be in the thorium or in an alloy diffusion layer and not a failure of the electroplated interface. The zinc electroplates were intact at 400 C., only 19 C. below the melting point of zinc.

While the method of the present invention has been generally described, it will be apparent to those skilled in the art that various modifications may be made with out departing from the scope of the present invention. Therefore it will be understood that this invention is not to be limited by the examples of the present specification, but only by the appended claims.

What is claimed is:

l. The method of pretreating a thorium surface prior to electroplating which comprises vapor blasting the surface, anodically pickling in a 5 to 15% by volume aqueous hydrochloric acid bath with a current of to 250 amperes per square foot for 3 to 5 minutes at room temperature, chemically pickling the surface in a 5 to 15% by volume of aqueous sulfuric acid for 3 to 5 minutes at room temperature, and then rinsing the surface with water.

2. In a method for electroplating a thorium surface the steps which comprise 'vapor blasting the surface, anodically pickling the surface in a 10% by volume aqueous hydrochloric acid bath with a current of 125 to 250 amperes per square foot for 3 to 5 minutes at room temperature, chemically pickling the surface in a 10% by volume aqueous sulfuric acid bath for 3 to 5 minutes at room temperature and then rinsing the thorium surface with water.

No references cited. 

1. THE METHOD OF PRETREATING A THORIUM SURFACE PRIOR TO ELECTROPLACING WHICH COMPRISES VAPOR BLASTING THE SURFACE, ANODICALLY PICKLING IN A 5 TO 15% BY VOLUME AQUEOUS HYDROCHLORIC ACID BATH WITH A CURRENT OF 125 TO 250 AMPERES PER SQUARE FOOT FOR 3 TO 5 MINUTES AT ROOM TEMPERATURE, CHEMICALLY PICKLING THE SURFACE IN A 5 TO 15% BY VOLUME OF AQUEOUS SULFURIC ACID FOR 3 TO 5 MINUTES AT ROOM TEMPERATURE, AND THEN RINSING THE SURFACE WITH WATER. 